Prolactin (PRL), an anterior pituitary hormone, has profound effects on female fertility. Hypersecretion of PRL is the major neuroendocrine-related pathology associated with female infertility. Isolated PRL deficiency, while rare, also results in reduced fertility and lactational failure in women. The underlying defects which lead to the common hypersecretion of PRL remain ill-defined, as do many of the mechanisms involved in the normal regulation of episodic PRL release. Usually, PRL secretion is under tonic inhibition by hypothalamic dopamine (DA). Periodic, physiological surges of PRL in females require a withdrawal from this dopaminergic inhibition. The fact that reduced responsiveness to DA is a hallmark of hyperprolactinemic syndromes demanded that we focus our attention on the cellular and molecular basis of DA action on the lactotrope. We have discovered and extensively characterized a DA-activated inwardly-rectifying potassium channel (KDA) in normal female lactotropes. DA regulates lactotrope membrane excitability through activation of KDA and in vitro studies have demonstrated a critical role for this KDAchannel in the regulation of PRL release by DA. Furthermore, the functional expression of KDAis dependent upon estrogen, which may explain a long-recognized modulatory action of estrogen on this system. In these regards, the KDA channel occupies a primal position in the hierarchy of events in dopaminergic regulation of PRL secretion. We have proposed to evaluate the role of this effector K channel within its physiological context - the whole animal through transgenic technology. To this end, we have designed and constructed "dominant negative" mutants, capable of inhibiting wild-type KDA function in lactotropes. We then created a transgenic mouse model ("Loss of Function") in which expression of this mutant is directed to pituitary lactotropes using the PRL promoter. A second transgenic model ("Gain of Function") has also been produced in which a constitutively active K channel, with the same biophysical properties as KDA, is over expressed in the lactotropes. Both models exhibit altered PRL secretory activity. These two models will enable us to study the physiological role of KDAand membrane excitability in the regulation of both basal and episodic release of PRL. Such studies will ultimately provide insight into the basis of secretory disorders in PRL, a hormone at the core of mammalian fertility, gestation and lactation.